The present invention relates generally to the art of compressing a gas. More particularly, the present invention relates to the compression of a refrigerant gas. Further, the present invention relates to the compression of a refrigerant gas in an oil-injected rotary screw compressor. With still more particularity, the present invention relates to apparatus in an oil-injected screw compressor for varying the capacity of the compressor and for separating oil from the refrigerant gas-oil mixture discharged from the compressor. Finally, the present invention relates to a slide valve assembly the actuating portion of which is integral with an oil separator located downstream of the discharge port in an oil-injected screw compressor.
Compressors are used in refrigeration systems to raise the pressure of a refrigerant gas from a suction to a discharge pressure which permits the ultimate use of the refrigerant to cool a desired medium. Many types of compressors, including rotary screw compressors, are commonly employed to compress refrigerant gas in refrigeration systems. Two complementary screw rotors, a male and a female, are located within a working chamber within the housing of a screw compressor. The working chamber can be characterized as a volume generally in the shape of two parallel intersecting cylindrical bores closely toleranced to the pair of meshed male and female screw rotors disposed therein. The screw compressor housing has low and high pressure ends defining suction and discharge ports respectively. Refrigerant gas at suction pressure enters the compressor suction port at the low pressure end of the compressor housing and is there enveloped in a pocket formed between the rotating complementary screw rotors. The volume of the gas pocket decreases and the pocket is displaced to the high pressure end of the compressor as the rotors rotate and mesh within the working chamber. The gas within such a pocket is compressed, and therefore heated, by virtue of the decreasing volume in which it is contained, prior to the pocket's opening to the discharge port at the high pressure end of the compressor. The pocket, as it continues to decrease in volume, eventually opens to the compressor discharge port at which point the compressed gas is discharged from the working chamber of the compressor.
One advantage of rotary screw compressors resides in the ability to easily modulate their capacity and therefore the capacity of the system in which the screw compressor is employed. Such capacity variance is normally accomplished through the use of a slide valve assembly. The valve portion of the slide valve assembly is built into and forms an integral part of the rotor housing of a screw compressor. Surfaces of the valve portion of the slide valve assembly generally cooperate with the remainder of the compressor's rotor housing to define the working chamber within the compressor. The slide valve is axially movable to expose a portion of the working chamber of the compressor, downstream of the suction port and which is not normally exposed to suction pressure, to a location within the compressor, other than at the suction port, which is at suction pressure. The portion of the working chamber initially opened to suction pressure by movement of the slide valve is that portion immediately downstream of the point at which compression of the refrigerant gas would normally begin within the working chamber. As the slide valve is opened further, a greater portion of the working chamber and the screw rotors therein are exposed to suction pressure. Capacity reduction is obtained by effectively reducing the portion of each rotor used for compression. When the slide valve is closed the compressor is fully loaded and operates at full capacity to compress refrigerant gas. When the slide valve is fully open, that is, when the portion of the screw rotors axially exposed to suction pressure other than at the suction port is greatest, the compressor is unloaded to the maximum extent possible. Positioning of the valve between the extremes of the full load and unload positions is accomplished without difficulty with the result that the capacity of a screw compressor, and the system in which it is employed, is modulated smoothly and efficiently over a large operating range. The slide valve is most often hydraulically operated.
Screw compressors used in refrigeration applications will, in the large majority of instances, include an oil-injection feature. Oil is injected into the working chamber of the compressor, and therefore into the refrigerant gas being compressed between the rotors therein, for several reasons. First, the oil injected into the working chamber acts as a sealant between the meshing screw rotors and between the rotors and the surface of the working chamber in which the rotors are disposed. Second, the oil acts as a lubricant. One of the two rotors in the screw compressor is normally driven by an external source, such as an electric motor, while the other rotor is driven by virtue of its meshing relationship with the externally driven rotor. The injected oil prevents excessive wear between the driving and driven rotors. Finally, in some applications, oil which has been cooled to increase its viscosity and its ability to act as a sealant is injected into the working chamber to cool the refrigerant undergoing compression therein which in turn allows for tighter rotor clearances at the outset.
Oil injected into the working chamber of a screw compressor is atomized and becomes entrained in the refrigerant gas undergoing compression therein. Such oil, to a great extent, must be removed from the oil-rich mixture discharged from the compressor in order to make the oil available for, among other things, reinjection into the compressor for the purposes enumerated above. Further, removal of excess injected oil must be accomplished to insure that the performance of the refrigerant gas is not unduly affected within the refrigeration circuit.
Previously, oil separation and slide valve actuation schemes have essentially been both structurally and functionally unrelated within screw compressor assemblies. Such disassociation has resulted in relatively complex and dedicated slide valve apparatus entirely separate from the oil separation apparatus within screw compressors. At worst, the two functions and their related structure are entirely disassociated within a compressor assembly. At best, the functions are only peripherally related within a compressor assembly. The former is illustrated by U.S. Pat. No. 4,335,582 while the latter is illustrated by U.S. Pat. No. 4,478,054. The disassociation of such apparatus within screw compressors exists despite the fact that in most instances both apparatus relate directly to the processing and use of oil within the screw compressor assembly. Whereas the separator functions to separate oil from the refrigerant gas-oil mixture discharged from the compressor in order to allow the oil to be reused, the slide valve assembly, in most instances, is actuated by such oil. Clearly, it would be advantageous to combine the slide valve assembly/oil separator functions to the extent possible within a screw compressor assembly to eliminate unnecessary duplication of structure, expense and weight. Until the apparatus of the present invention was conceived, no integral slide valve assembly-oil separation scheme for screw compressors was known to exist.